TECH Q: Whats a De Dion and whats a live axle etc?
Discussion
Live Axle - Beam axle, ridged with the diff mounted in it. Like the Escort Mk 1 and 11.
De Dion, Has the diff mounted on the chassis with drive shafts going out to stub axles at the end. The De Dion part is a solid beam that runs from left to right so that the wheel act in the same way as a live axle through cornering. ie if the left wheel goes up, the right goes down, does that make sense?
Advantages of the beam axle - Simple and the wheels stay perpendicular with the road surface as the body rolls. Disadvantage, heavier unsprung mass, meaning the dampers and springs are less sensitive.
De Dion, lighter unsprung mass, meaning that better ride and handling can be achieved with springs and dampers. Disadvantage, general weighs slightly more than a standard live axle set up.
Hope that helps,
Mark
De Dion, Has the diff mounted on the chassis with drive shafts going out to stub axles at the end. The De Dion part is a solid beam that runs from left to right so that the wheel act in the same way as a live axle through cornering. ie if the left wheel goes up, the right goes down, does that make sense?
Advantages of the beam axle - Simple and the wheels stay perpendicular with the road surface as the body rolls. Disadvantage, heavier unsprung mass, meaning the dampers and springs are less sensitive.
De Dion, lighter unsprung mass, meaning that better ride and handling can be achieved with springs and dampers. Disadvantage, general weighs slightly more than a standard live axle set up.
Hope that helps,
Mark
Mark's got it. For a more modern example of a live axle, look under any truck, or a RWD van.
Imagine how heavy that axle is. All that weight is "unsprung" mass (it's not supported on the springs). For every bump and ripple in the road, that lump has to be moved up and down. Tyre load varies enormously (=worse handling), the springs and dampers have to control that moving mass (and therefore transfer more loads into the chassis = worse ride).
General theory is that you want to minimise unsprung mass. The de Dion system gives you the benefits of a beam axle (no geometry changes), but moves much of the weight into the sprung mass.
In theory it should imporve comfort, and improve roadholding, by riding the bumps better. Not having driven both versions, I can't comment directly, but my recollections are that the de Dion is a bit more comfortable. On track (where there are very few bumps), performance is about equal. But on the road, the de Dion feels much better
Imagine how heavy that axle is. All that weight is "unsprung" mass (it's not supported on the springs). For every bump and ripple in the road, that lump has to be moved up and down. Tyre load varies enormously (=worse handling), the springs and dampers have to control that moving mass (and therefore transfer more loads into the chassis = worse ride).
General theory is that you want to minimise unsprung mass. The de Dion system gives you the benefits of a beam axle (no geometry changes), but moves much of the weight into the sprung mass.
In theory it should imporve comfort, and improve roadholding, by riding the bumps better. Not having driven both versions, I can't comment directly, but my recollections are that the de Dion is a bit more comfortable. On track (where there are very few bumps), performance is about equal. But on the road, the de Dion feels much better
HiRich said:True but it does assume that the road is the same for both wheels (and flat, or that the bumps hit both wheels at once). If you're hitting rumble strips with the inside wheel you don't want that transfered to the outside wheel by the de-dion beam, same goes for raod driving and potholes
In theory it should improve comfort, and improve roadholding, by riding the bumps better.
I think
My comment on the Caterfields was only based on slose examinations of westfield chassis, just guessed they'd be the same, sorry
No need to appologies
The De Dion setup is a comprem ise between a live axle and a fully independent arrangement. One of it's benefits however is that it's more compact than a fully IRS as you don't need upper wishbones. So for a Caterham (which is significantly smaller than a Westfield the IRS is not an option).
The De Dion setup is a comprem ise between a live axle and a fully independent arrangement. One of it's benefits however is that it's more compact than a fully IRS as you don't need upper wishbones. So for a Caterham (which is significantly smaller than a Westfield the IRS is not an option).
Incorrigible said:
True but it does assume that the road is the same for both wheels (and flat, or that the bumps hit both wheels at once). If you're hitting rumble strips with the inside wheel you don't want that transfered to the outside wheel by the de-dion beam, same goes for raod driving and potholes
Not if your comparing de Dion with a live axle. Even on single-wheel bump, there is less mass to be moved, so the tyre can map the road better.
Comparing de Dion with independent is really comparing two different tachnologies. Independent (on a Caterfield) should be better all round:
- Less unsprung mass
- More control over camber, toe, pitch & dive, and roll
but that assumes you have a decent setup (like decent length wishbones running close to the centreline of the chassis). A beam axles/de Dion gives less true camber change in bump & roll (though still some, as the loaded tyre compresses and deforms, so the axle does not stay parallel to the road), but a good independent system can actually make use of the variation in camber to benefit roadholding.
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